How NOT to De-ice Your Windshield

Thanks for stopping by Be Car Chic. I hope you’ll bookmark my site for car reviews and auto advice. This post was written during a bitter winter a few years ago, but every time the temps drop, nice readers like you land here for tips on how to properly de-ice your windshield. To make your visit quick and helpful, here’s the skinny:

If it’s really frigid outside, then pouring hot water on your windshield is not a good idea. The temperature difference could cause the windshield to crack. If it’s not that cold, then warm water may help the ice melt.

A chemistry expert from About.com says dense heat conductors, such as your hand, works well on the inside of the windshield to soften the ice on the exterior. Of course, if there’s a thick layer, that method could take a while.

At the end of the day, the best method remains your good ole ice scraper!

The comments are also chock full of helpful tips from others on this same challenge.

(Updated November 2014)

Last night, the DC area had a wintry mix that left exposed vehicles with an icy coating. I returned from an appointment early this morning, and I pulled into the parking lot where I saw the gent who would inspire this post.

I’m fortunate to have a covered spot; therefore, the icy conditions didn’t delay my commute. When I had left the lot for my appointment, there was a guy scraping the windshield of his Toyota Celica…30 minutes later when I returned, he was still there, but now pouring a bucket of steaming hot water on his back windshield. (Everybody together now, “GASP!!”)

I thought, “Well that’s not very smart.” Then I rolled down my window and politely said, “Hey, just so you know, that can crack your windshield.” He didn’t seem to understand what I meant so I reiterated the point: “Hot water can cause the glass to crack.” A look of sheer surprise came over his face and then a look of guilt, as though I had caught him with his hand in the cookie jar. (To be fair, his reaction could have been because I may have used my “Mom” tone.)

He had nearly finished scalding his windshield and nothing shattered. I tried to comfort him by saying that he’s probably fine this time, but he wouldn’t want to do that in the future. I felt a little bad for the guy, but hey, he’ll know better next time! Consider it my good will act of the day.

This leads me to the entire purpose of this post. The take-away is as simple as these two, measly sentences on how NOT to de-ice your windshield:

Do not pour hot water on a frozen or iced-over windshield. The temperature difference can cause the glass to crack.

You’ve woken up after watching that clip but still aren’t convinced. Perhaps a little scientific evidence will help. OK, the term is: “thermal shock.” I’m no scientist, but allow me to try to explain.

Say your car has a few minor dings or cracks in the windshield – typical damage from rocks and debris when on the highway. Well, those specks may seem minor, but they can cause major problems if you do not heed this advice.

It’s 20 degrees outside, and your windshield is completely iced over from the wintry mix the night before. You pour hot water onto that below-freezing-temperature glass and it seeps into those small dings/cracks and freezes upon impact. And we all know that water expands when it freezes. So now it’s in the crack of your windshield and it is growing with nowhere to go. This causes stress to the glass around it, resulting in a cracked windshield.

If that’s not enough motivation, then, for those of you trying to keep your girlish figures out there – scraping off ice the old-fashioned way also burns calories.

I don’t think pouring hot water on your windshield cracks the windshield because it seeps into the small cracks, freezes and expands. It is because of the extreme temperature change of the glass itself. When glass is heated, it expands. That is what causes the crack.

A very helpful post. The thermal shock the reason. Although I see the physics behind it differently. The auto-glass is not a wafer thin layer but quite thick and the windshield is made of multiple layers. Now the ultra-fast heating (and expansion) of the surface while the deeper/inner part of the glass is still cold creates the cracks.
Metals(unlike glass) can handle this kind of thermal shock because they are malleable and conduct heat much faster(easier) than glass can. So feel free to dump scalding water on your car’s hood on a frozen night

Thanks for this. You do conflate/confuse two different ways water could cause the glass to crack. One is thermal shock or the thermal differential. That happens when something is entirely cold, and part of it is warmed up (or vice versa, actually.) The warmed part tries to expand but the cold part doesn’t, so stress builds up between the two parts, possibly causing it to crack. So for a windshield, the hot water would warm up some of the glass, perhaps just the passenger side, say, or perhaps the outside surface of the glass. The hotter the water or the colder the glass, the more likely it is to crack. The second is freeze cracking, where water freezes in the existing tiny cracks and pits in the glass. In the second, it doesn’t matter how hot the water is. As long as it is liquid, water could theoretically get into the cracks, freeze, then expand, widening the crack.

I could easily believe the first phenomenon can and does happen, especially since safety glass has built-in stress, so it shatters into tiny pieces when broken. One way to help prevent it would be to use a lot of water over the entire surface of the windshield, so that all of it warms up at the same time. The second phenomenon I am more doubtful about. It does happen with concrete, and indeed, it is a big issue for sidewalks and driveways. But on my windshield, there is a visible crack but it is so tight that I can’t feel it. It is easily airtight, and I am pretty sure no water actually gets inside it. Anyway, for that mechanism to occur, the water would have to hit the ice-covered glass, reach the glass, and then seep into the cracks, and meanwhile the glass has to have stayed cold enough to freeze the water. It seems unlikely. I could see it happening much more easily if there is a long rain that turns into ice and snow overnight. That way the glass is soaking wet, and maybe a tiny bit could seep into a crack and then freeze as temperatures drop.

Anyway, a simple way to clear the ice is to use room-temperature water with salt or sugar in it, anything to lower the freezing point. The salt solution will warm the ice without itself freezing, and by running off the ice, it carries the coldth away.

Been doing it for decades- no prob. I live on Vancouver Island but it can get to minus 10 C about 0 F. I use warm to hottish water not boiling- why bother.
Would it be safe at minus 40 C=minus 40 F. Don’t know but I do know this scare talk at temps above is BS. I’ve done this a minimum of several thousand times over 30 years, it was just routine.
BTW- I’ve just spent about half an hour trying to find video of this happening. There are lots of junk yard clunkers to demonstrate this on.

Trackbacks/Pingbacks

[…] in 2010, we put together five tips to winterize your car. That post, and our advice on how NOT to de-ice your windshield have been some of our most popular winter-themed posts. Today, we bring you some additional tips […]

[…] MASON The temperature difference between the scalding water and your freezing car may crack your windshield. Remember that rock that put a lovely crack in the glass yesterday? The hot water will work its way […]